Given below are two statements. One is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$ : If $dQ$ and $dW$ represent the heat supplied to the system and the work done on the system respectively,then according to the first law of thermodynamics $dQ = dU - dW$.
Reason $R$ : The first law of thermodynamics is based on the law of conservation of energy.
In the light of the above statements,choose the correct answer from the options given below:

• A
  $A$ is correct but $R$ is not correct.
• B
  $A$ is not correct but $R$ is correct.
• C
  Both $A$ and $R$ are correct and $R$ is the correct explanation of $A$.
• D
  Both $A$ and $R$ are correct but $R$ is not the correct explanation of $A$.